The present invention relates to a method for suppressing undesired control actions in slip-controlled brake systems, which, for example, may be caused by oscillations of the undercarriage or the axle. The method determines the wheel rotational behavior and controls the braking pressure as a function of slip criteria and/or acceleration criteria and which initiates the slip control when predetermined slip and/or wheel acceleration threshold values are exceeded. Circuit configurations for implementing this method are followed from the present invention.
In vehicles incorporating brake systems with slip control, the rotational behavior of the individual wheels are constantly measured, and further braking pressure rise will be stopped or braking pressure will be decreased on detection of wheel slip of deacceleration values indicative of an imminent locked condition and when specific predetermined threshold values have been exceeded. As soon as the wheel rotational behavior permits, the braking pressure will be re-increased in order to ensure the transmission of the highest braking pressure possible or, respectively, braking action affording a short stopping distance.
To attain an optimal braking behavior, there is need for a control which affords an optimally sensitive and swift reaction on changes in the wheel rotational behavior announcing an imminent locked condition. However, for elimination of inevitable errors and tolerances in the measuring system, in the signal processing etc., the control should not respond until specific values are exceeded.
In addition, oscillations may occur in the wheel suspension under certain conditions which cause relatively great differences between the vehicle speed and the wheel speed as well as positive and negative wheel acceleration values, which falsely indicate an unstable run of the wheel and initiate control actions. That is, sudden variations in the driving torque of a vehicle wheel may bring about excitation of damped oscillations in the drive shaft line--in the undercarriage or in the wheel suspension system--which oscillations are known to be subject to the differential equation for oscillations ##EQU1## where X represents the oscillation amplitude and X.sub.o represents the initial value of the oscillation (t=0) and .sigma. represents a factor dependent upon the frictional force and the oscillating mass. Such variations of the driving torque take place constantly during control of a vehicle wheel. This is primarily caused because in the initial phase of braking pressure build-up, in which the brake torque is preponderant, the resilient elements of the undercarriage will be preloaded and will be relieved from load abrupty during the phase of braking pressure decrease. As a result thereof, the sensor fitted to the axle and serving to determine the rotational behavior will perform a relative movement towards the sensor wheel co-rotating with the vehicle wheel and will image the decaying oscillation in the speed range. Therefore, evaluation of the sensor signals will recognize speed differences and variations which may initiate control cycles although the vehicle wheel is running stably. Such control actions caused by misinterpretations have adverse effects on the energy consumption of the brake system, on the control comfort and on the braking effect. In the worst case, the undercarriage oscillation may even "build up". Abrupt changes of the frictional values may likewise lead to conditions similar to those caused by oscillations. Furthermore, the vehicle wheels can be alternately deaccelerated and accelerated on so-called bumpy roads. There is the imminent danger that the electronic controller will interpret the deacceleration periods as control periods and will erroneously initiate pressure reduction. It is therefore the object of the present invention to arrange for a method which suppresses unwanted control actions which may be caused in particular by undercarriage or axle oscillations, without decreasing the response sensitivity and the rapidity of the control as a reaction to tendencies to lock.